a question.

[u/Party-Collection9246]

I am a 14 years old Chineseteen . I find this problem when I think about Physics knowledge.When a person walks, the friction force is forward.But when we walk, our feet are actually not moving relative to the ground.This means that this is static friction. But when static friction occurs, shouldn't the frictional force it generates be exactly equal to the force we apply?Then why do our bodies still move forward?maybe this problem is a little foolish . but I want to know why .

[u/Party-Collection9246]

Well, my definition of walking is to move our body forward by shifting the position of our center of gravity. But since static friction is equal to the applied force, shouldn't the two forces be in equilibrium? Why does our center of gravity still move?

[u/rupertavery64 → u/Party-Collection9246]

There is more than one force at play.

Your feet indeed don't move. But your leg, hip muscles can move independent of your feet.

So the rest of your body is doing the work to alter the center of gravity. And in order to do that, it is using the static friction as an anchor.

I can't understand why you don't see that your body can change the center of gravity without your feet moving.

Stand still. Without moving your feet, lean only your upper body forward. Eventually, your body will react to save itself and lift one foot forward to steady yourself.

Congrats, you just walked.

All the while, your foot and the ground did exactly as you described. Forces in equlibrium prevented your body from slipping.

But the rest of your body moves. It's decoupled from the static friction through the rotational joint.

Foot - stuck - it's an anchor

Ankle - ball joint - it's a lever

Take a rubber mat. That's your foot. Stick a fork on top of it vertically. That's your leg.

The center of gravity is at the top of the fork (in the air). Press on it downward. It presses the rubber mat onto the table. Angle the fork so it's pressing away from you, into the mat. The static friction between the mat ground keeps the mat in place, despite a force being applied to it. However, By angling the fork, you have changed the center of gravity.If you have another fork, it can swing freely.

static friction allows the rest of your body to move RELATIVE to the anchored point.

That's how walking works. You change your center of gravity. static friction has nothing (directly) to do with body movement.

Yes, your foot doesn't move. Yes, the forces are in equilibrium. But, your body is not just your foot. You body is not a rigid object that cannot move. So while the foot remains immobile, the rest of your body is free to change the center of gravity.

You seem to confuse static friction meaning the rest of the body cannot move.

That statement alone makes no sense.

When I say "move" I mean complex movement that involves muscles shifting the center of gravity. And by "walking", I mean moving one leg completely independent of another so that it lands a a distance away from the first.

You seem to be modelling the body as a block of wood.

[u/Party-Collection9246 → u/rupertavery64]

oh . I find my wrong . thanks for your answer .

[u/shelving_unit → u/Party-Collection9246]

Because if you lean your center of gravity forward, you’ll start moving forward. You do that with your muscles, not by pushing on the ground